1. Field of the Invention
The present invention relates to a transfer device, and an image forming apparatus.
2. Description of the Related Art
Tandem-drum image forming apparatuses and single-drum image forming apparatuses have been known. For example, Japanese Patent Application Laid-Open No. 2004-142920 discloses a tandem-drum image forming apparatus that includes a plurality of image carriers, such as photosensitive members. The image carriers are brought into contact with a surface of an endless intermediate transfer belt to form a plurality of primary transfer nips. In one of the primary transfer nips at which a first primary transfer process is performed, a toner image on an image carrier is transferred onto a surface of the intermediate transfer belt on which no image is transferred yet. In contrast, for the other primary transfer nips, a toner image on the image carrier is primary-transferred onto the already-transferred toner image on the intermediate transfer belt to thus be superimposed thereon. Through such a superimposing primary transfer process, a superimposed toner image is formed on the intermediate transfer belt. The superimposed toner image is collectively secondary-transferred onto a recording medium (e.g., recording sheet) nipped in a secondary transfer nip formed as a contact portion between the intermediate transfer belt and, e.g., a roller.
On the other hand, Japanese Patent Application Laid-Open No. 2004-109575 discloses a single-drum image forming apparatus that includes only one image carrier. The image carrier is brought into contact with a surface of an endless intermediate transfer belt to form a primary transfer nip. During a period in which the intermediate transfer belt rotates a plurality of times, toner images formed on the image carrier are transferred onto the intermediate transfer belt to thus be superimposed one upon another on the intermediate transfer belt. When a superimposed toner image is formed on the intermediate transfer belt, a shifting mechanism that brings a roller member or the like into and out of contact with the intermediate transfer belt is actuated so that the roller member is brought into contact therewith to form a secondary transfer nip. The superimposed toner images on the intermediate transfer belt are collectively secondary-transferred onto a recording medium nipped in the secondary transfer nip.
In the conventional technologies described above, toner images superimposed in multiple layers pass through the primary transfer nip in the primary transfer process. This poses a problem that an overpressure is undesirably applied to the multi-layered toner images, which induces a defect related to superimposing transfer such as a void.
Moreover, moisture absorption by a recording medium is likely to induce defective secondary transfer. More specifically, a recording medium made of fiber or the like absorbs moisture under an environment of high temperature and high humidity, resulting in less electric resistance across the recording medium. When the recording medium of the thus-decreased resistance is nipped in the secondary transfer nip formed as a contact portion between the intermediate transfer belt and a roller member to which a secondary transfer bias is applied, a transfer current undesirably flows to the ground via the roller member, the recording medium, a guide member contacting the recording medium, and the like. Thus, the transfer current supplied from the roller member to the intermediate transfer belt becomes undercurrent, thereby causing defective secondary transfer.